This invention relates generally to boilers and more particularly to improvements in boilers of the type in which gas and air are premixed in the desired proportions and then supplied to the burner for combustion.
Gas boilers typically have cylindrical burners of the general construction shown in U.S. Pat. No. 3,936,003 to Hapgood et al. Due to the cylindrical shape of conventional burners, they are subject to various problems, most notably non-uniform flame distribution and overheating of the end opposite the inlet end of the burner. Because the cross-section of a cylindrical burner is constant and the fuel mixture is introduced at one end, the flame distribution varies considerably along the burner length, thus reducing the efficiency and the burner life. Although baffles have been used in burners to attempt to remedy the uneven flame distribution, baffles have not been able to achieve significant improvements. In addition, the need to provide baffles complicates the burner construction and increases its costs significantly.
In order to prevent the fuel mixture from passing largely through the end opposite the inlet end, cylindrical burners provide a closed or imperforate end. Consequently, the closed end becomes red hot during normal firing of the burner and an undesirable "hot spot" thus develops. The extreme heat to which the closed end is subjected can cause it to burn through or otherwise fail prematurely, and the boiler efficiency is also reduced.
Another problem with cylindrical burners is that the fabrication process is complicated because seam welding is required not only along the longitudinal seam but also at the closed end. A final problem is that the flame exhibits instability because the burner ports are arranged uniformly and the flame propagation rate or ignition velocity cannot be exceeded appreciably without creating flame instability.
Gas boilers are somewhat notorious for noise problems that arise principally from the phenomenon of combustion noise created by oscillations or pulsations in the combustion chamber coupled with pressure fluctuations in the burner fuel supply system. The combustion oscillation is characterized by a standing wave at a specific frequency in the combustion chamber. If the phase of the standing wave is such that the air/fuel supply is modulated in phase with it, the pulsation pressures are amplified and the noise is particularly objectionable. The presence of a standing wave in the flue passage provides the feedback mechanism for oscillations that generate noise. However, efforts that have been made in the past to inhibit or destroy the standing wave have created significant decreases in the boiler efficiency, and one problem is merely substituted for another if efficiency is sacrificed for the sake of noise reduction.
Another noise problem can be caused by the blower wheel which supplies the fuel-air mixture to the burner. The blower wheel cannot be balanced perfectly and some imbalance must be accepted and dealt with as a practical matter. If the blower is out of balance and the motor that drives it is mounted rigidly, vibrations are created and objectionable noise can be generated. Therefore, the motor is normally mounted resiliently so that the vibrations and noise are eliminated or at least suppressed to an acceptable level. However, the resilient mounting provides the motor shaft with side to side play, and the hole in the blower housing through which the motor shaft or blower hub extends must be oversized in order to accommodate the play that is permitted. This creates a source of air ingress into the blower housing around the shaft or hub, and the air which is drawn into the blower housing ca dilute the gas/air mixture enough to create adverse effects on the combustion process.